Safety circuit



May 14, 1963 E. J. CAMFIELD ETA; 3,089,985

SAFETY CIRCUIT Filed Oct. 27, 1960 PBI TDEA garb* CEIA Totem die? /NVE/v T095 EJCAMF/ELD K/.EH/)USCH/LD E Mme/v Y United States Patent O York Filed Oct. 27, 1960, Ser. No. 65,463 4 Claims. (Cl. 317-135) This invention relates to electrical circuitry for actuating a load device only upon substantially simultaneous actuation of two manually operable switches, whereby the load device is operable only, for example, when the operators hands are safely removed from a danger zone. It is an object of the invention to provide improved electrical circuitry of thi-s character.

Prior art circuitry serving the basic function of preventing operation of a load device, such as la punch press or molding machine, except when two manually oper-able switches are operated concurrently, has had vari-ous disadvantageous characteristics whereby it has been less than fully satisfactory as safety means. Such prior art circuitry has in various instances permitted the jamming of one switch in its actuated position such that only the remaining switch must be manually operated to erec-t operation of the machine. It the circuitry permits such jamming of one switch, it is ineffective as a safety circuit. Other prior art circuitry has been unsatisfactory because it falls short oi being fully safe Kfor other reasons, because of a lack yof reliability, and/or because of requiring expensive circuit components.

Circuitry which is in accordance with the present invention employs an inexpensive form of time delay relay in each of the two control circuits containing the two manually operable switches, the time `delay relay in each of the two control circuits being adapted, a prescribed period of time after initiation thereof, to prevent completion of the other control circuit, but being ineffective to lopen the other control circuit after it 'has been actuated by virtue ot a lock-in circuit. Accordingly, both control circuits can be actuated only if both manually operable switches `are operated prior to the effective operation of either time delay relay, but, when thus actuated, both control circuits are independent of subsequent oper-ation of the time delay relays. Both control circuits must be actuated to complete the load circuit, that is, to actuate the load device.

Accordingly, it is another object of the invention to provide improved electrical circuitry of the character described ahove which prevents actuation of a load device after one of two manually operable switches has been jammed in its actuated position.

It is a further object of the invention to provide improved electrical circuitry of the character described above which fails safe upon failure of applied electrical power.

It is still another object of the invention to provide improved electrical circuitry of the character described above which employs conventional and inexpensive components and which is efficient and reliable in operation.

Other objects and advantages of the invention will become apparent by reference to the Ifollowing description and the accompanying drawings illustrating a preferred embodiment thereof, in which- A FIG. l is a circuit diagram illustra/ting the preferred embodiment of the invention, and

FIG. 2 is a similar circuit diagram illustrating another embodiment of the invention.

As indicated above, electrical circuitry incorporating the present invention is particularly applicable to machines whose opera-tion comprises a hazard to the opera- ICC tor, particularly in that one of the operators hands might be hurt. 'Ihe provision of such electrical circuitry as herein described requires that the operators hands be substantially removed from the danger zone before operation of the machine can be effected. Conventionally, such circuitry employs push buttons such as those designated PBI Eand PBZ in FIG. '1. It will be understood by those skilled in the art that these push buttons are arranged at a substantial distance trom the danger zone of a machine and are suthciently spaced trom each other that they cannot be operated by one hand. This being the case, the operators hands must, at one time, be removed `from the danger zone in order to actua-te the two push buttons. However, it is also essential that the electrical circuitry require substantially simultaneous operation of the two push buttons since the operator might otherwise depress one push button and then move one hand into the danger zone before he depresses 4the other push button. More commonly, an operator will jam one switch closed so that he may operate the machine by depression of the remaining push button. The circuitry illustrated in FIG. l prevents machine operation under these circumstances and thereby eliminates the danger of injury :to the operator from the-se causes.

It may be seen that push button PBI controls the ow of current between two electric powerlines A and B to a conventional control relay CRI and a Itime delay relay TDRI. The other push button FB2 controls the ilow of current through a conventional relay CR2 and a time delay relay TDRZ. The connection between the push button PBI and relays CRI and rl`DR1 is normal-ly completed by contacts TDRZA which are normally closed contacts of the time delay relay TDRZ ot the second control circuit. It will be appreciated, therefore, that if the push button PBZ is closed substantially ahead of the push button PBI, such that the time delay relay TDRZ reaches 'its `fully actuated condition before the push button PBII is depressed, the contacts TDRZA will have opened the tirs-t control circuit such that closure of the push button PBI will not cause energization of the relays CRI and TDRI. Similar normally closed contacts TDRIA, of the time delay relay TDRI of the rst control circuit, are arranged in the second control circuit. Accordingly, if the push button PBI is depressed substantially ahead of the push button PE2, the contacts TDRIA will have opened the second control circuit such that the depression of the push button PE2 will not etl'ect energization of the relays CR'Z and TDRZ.

Returning again to the iirst control circuit, contacts CRIA are arranged in a bypass circuit around they contacts TDRZA. The former are normally open contacts of the conventional relay CRI. With this addition to the iirst con-trol circuit, it will be seen that after the relay CRI has been actuated, the resulting closure of the contacts CRlA bypasses the contacts TDRZA. Accordingly, when the time delay relay TDRZ of the second control circuit reaches its actuated position, the opening of its contacts TDR2A in the iirst control circuit will have no eitect on the iirst control circuit.

Similarly, normally open contacts CRZA of the relay CRZ are arranged in a bypass circuit around the contacts TDRIA of the second circuit. Accordingly, actuation of the relay TDRI of the first circuit after actuation of the conventional relay CR2. of the second circuit does not open the second circuit.

The lower circuit of FIG. 1 is the power or load circuit, current being allowed to pass through Ithe indicated load device when both 4ot two switches `CRIB and CRZB are closed concurrently. These last-mentioned switches are normally open contacts, respectively, of the relay CRI and the relay CRZ. It will Ibe noted that both of the two control circuits must be energized before the load can be energized.

The circuitry of FIG. l now having been described, it is believed that a ful-l understanding of the invention will best be obtained following consideration of both improper and proper operation of the circuitry. Let it be assumed iirst that the time delay relays TDRI and TDRZ require approximately one-quarter second to reverse their contacts following initial energization. Let it now be assumed that the operator depresses PBI approximately one-half second before he depresses the push button PBZ. Immediately upon depression of the push button PB1, the relay CR1 is actuated and closes the contacts CRlA in the rst control circuit and the contacts CRlB in the power circuit. The time delay relay TDRl of the iirst control circuit will also be energized immediately, and after one-quarter second will have actuated its contacts TDRIA of the second control circuit. Since the push button PB2 has not yet been closed to energize the relay CRZ and close the relay contacts CRZA, the latter are open. With both the contacts TDRIA and the contacts CRZA open, the subsequent depression and closure of the push button PBZ is ineffective. Since the relay CRZ cannot be operated, the relay contacts CRZB in the power circuit cannot Ibe closed and, therefore, the load cannot be energized.

Similarly, depression of the push button PBZ one-half second Vbefore depression of the push button PBl will permit actuation of the time delay relay TDRZ and opening of the relay contacts TDRZA in the first control circuit such that the subsequent depression and closure of the push button PBl is ineffective. Since the relay CR1 cannot be energized and the relay contacts CRIB in the power circuit cannot be closed, the load device cannot be energized.

Since energization of the load device is dependent upon actuation of the two push buttons PBI and PBZ substantially simultaneously, that is, within a time period of approximately one-quarter second, it will be appreciated that the operator cannot effect operation of the load device -if he jams one of the push buttons in its actuated or closed position by means of a wedge, prop or weight.

In a proper operation of the circuitry, the two push buttons are depressed within a time period of approximately one-quarter second. Let it be assumed that the push button PBl is depressed first and that the push button FB2 is depressed one-eighth second after the depression of the push button PBl. The relay CRl will 'be actuated first and will close its contacts CRlA and CR1B. The time relay relay T DRI will also be energized but it will not actuate its contacts TDR1A unt-il after the lapse of one-quarter second. After the lapse of only oneeighth second, `the push button P-BZ is depressed in accordance with our assumption. The relay CRZ is immediately yactuated and closes its contacts CRZA and CRZB. When the time delay relay TDRl subsequently reaches its actuated condition, the opening of its contacts TDRlA is ineffective to open the second control circuit since those contacts have already been bypassed by the relay contacts CRZA. Still later, when the time delay relay TDRZ reaches its actuated condition, the opening of its contacts TDRZA is ineffective to open the Afirst control circuit since the relay contacts CRIA have already closed the bypassing circuit around the associated time delay relay contacts TDRZA.

During this sequence of operation, the load became energized upon the closing of relay contacts CRZB, the relay contacts CRIB having closed shortly before.

In the circuitry of FIG. l, it should be noted that only four relays are required. Two of these are conventional relays which operate substantial-ly instantaneously although it is not necessary that these relays be chosen to operate at unusually high speed. It is necessary only that these relays operate substantially faster than the time delay relays. The other two required relays, namely,

the two time delay relays, may be of commercial, economical and readily available form. They need not be adjustable, and preferably are not adjustable, since the operator might adjust an adjustable relay such that its prolonged time period Would render the circuitry ineffective as a safety measure.

In addition, the contacts required on the relays are simple in form and small in number. More particularly, .the time delay relays each require only a single normally closed set of contacts. The conventional relays CRI and yCR2 each require two sets of normally open contacts.

The embodiment of the invention illustrated in. FIG. 2 employs substantially the same components in substantially the same arrangement and accordingly are designated by the saine reference characters. Only two relatively minor changes are made over the circuitry of FIG. 1, and accordingly only those changes are described herein.

It will be noted rst that the power circuit at the bottom of the diagram employs normally open contacts TDRlB and TDRZB of the time delay relays TDRl and TDR2. This is a modification over the power circuit of FIG. 1 in which the contacts are normally open contacts of the conventional relays `CR1 and CRZ. The use of time delay relay contacts instead of conventional relay contacts to control the load device makes little difference except as to the rapidity of response of the circuitry. The circuitry of FIG. 1 will, of course, produce a more rapid response and may therefore be preferred in some instances.

The other change involves the use of normally open contacts CRIC of the relay CRI in series with the time delay relay TDRI, and normally open contacts CRZC of the relay CR2 in series with the time delay relay TDR2. As will be apparent to those skilled in the art, the effect of placing these normally opened contacts of the conventional relays in series with the corresponding time delay relays is to delay initial energization of the time delay relays until such time -as the associated conventional relay has been actuated. The two associated relays CRI and TDRI, for example, operate in sequence, the time `delay relay being initially energized only after the conventional relay has been actuated. This arrangement provides `for a permissible time delay between depression of the two push buttons which is equal to the delay period of the time vdelay relays. As opposed to It-his, the permissible time delay in the operation of the two push buttons in the circuitry of FIG. l is equal to the delay period of the time delay relays minus the operation time of the conventional relays.

It should be noted that in circuitry in accordance with the present invention, the two control circuits, each containing one of the manually operable switches, condition each other against actuation but only following the lapse of a predetermined period of time dictated by the time delay relay incorporated in each control circuit. In addition, each control circuit is ineffective to open the other control circuit if the other control circuit has become energized prior to the lapse of the prescribed time interval.

It will be apparent that circuitry in accordance with the present invention effectively prevents jamming of either manually operable switch and attendant loss of the safety factor. It should be noted also that loss of power to any of the three branches of the circuitry in either of the illustrated embodiments of the invention causes the circuitry to fail safe. That is, such power failure will result not in operation of the load device but in inability to operate Ithe load device. It will also be appreciated by those skilled in the 4art that only conventional, economical and readily available relays are called for by the circuitry, and that the circuitry is inherently eicient and reliable in operation.

It is to be understood that the above-described arrangeepesses ments lare simply illustrative of the application of the principles of the invention. Numerous other arrangements may be readily devised Eby those skilled in the lart which will embody the principles of the invention and fall Within the spirit and scope thereof.

What is claimed is:

l. Electrical circuitry for actuating a load device only upon substantially simultaneous actuation of two manually operable switches, comprising two control circuits and an associated output circuit, each of said control circuits including a control relay and a time delay relay arranged in parallel, and a normally open, manually closable switch arranged in series with said relays, each of said time delay relays having normally closed switch contacts arranged in series with the manually operable switch and the relays of the other control circuit, whereby each time delay relay, a prescribed period of time after closing of the associated manually operable switch, may prevent completion of the other control circuit, each of said control relays having normally open switch contacts arranged, when closed, to bypass said time delay relay switch contacts of its own control circuit, whereby actuation of one of said time delay relays may not open the other control circuit after actuation of the control relay thereof, said output circuit including, in series arrangement, a load device and two normally open switches, each of said switches being associated with one of said control circuits and being closable by actuation of one of the relays thereof.

2. Electrical circuitry for actuating a load device only upon substantially simultaneous actuation of two manually yoperable switches, comprising two control circuits and an associated output circuit, each of said control circuits including a control relay and a time delay relay arranged in parallel, and a normally open, manually closable switch arranged in ser-ies with said relays, each of said time delay relays having normally closed switch contacts arranged in series with the manually operable switch and the relays of the other control circuit, whereby each time delay relay, a prescribed period of time after closing of the associated manually operable switch, may prevent completion of the other control circuit, each of said control relays having normally open switch contacts arranged, when closed, to bypass said time delay relay switch contacts of its own control circuit, whereby actuation `of one of said time delay relays may not open the other control circuit after actuation of the control relay thereof, each of said control relays also having normally open switch contacts in series with the associated time delay relay, whereby actuation of the time delay relays may not begin until after the associated control relay has been actuated, said output circuit including in series arrangement, a load device and two normally `open switches each associated with one of said control circuits and closable by actuation of one of the relays thereof.

3. Electrical circuitry as speciiied in claim 1 wherein said normally open switches in said load circuit are closable by actuation of respective ones of said control relays.

4. Electrical circuitry as specified in claim 2 wherein said normally :open switches in said load circuit are closable by actuation of respective ones of said control relays.

References Cited in the iile of this patent UNITED STATES PATENTS 2,753,493 Saives July 3, 1956 

1. ELECTRICAL CIRCUITRY FOR ACTUATING A LOAD DEVICE ONLY UPON SUBSTANTALLY SIMULTANEOUS ACTUATION OF TWO MANUALLY OPERABLE SWITCHES, COMPRISING TWO CONTROL CIRCUITS AND AN ASSOCIATED OUTPUT CIRCUIT, EACH OF SAID CONTROL CIRCUITS INCLUDING A CONTROL RELAY AND A TIME DELAY REALY ARRANGED IN PARALLEL, AND A NORMALLY OPEN MANUALLY CLOSABLE SWITCH ARRANGED IN SERIES WTH SAID RELAYS, EACH OF SAID TIME DELAY RELAYS HAVING NORMALLY CLOSED SWITCH CONTACTS ARRANGED IN SERIES WITH THE MANUALLY OPERABLE SWITCH AND THE RELAYS OF THE OTHER CONTROL CIRCUIT, WHEREBY EACH TIME DELAY RELAY, A PRESCRIBED PERIOD OF TIME AFTER CLOSING OF THE ASSOCIATED MANUALLY OPERABLE SWITCH, MAY PREVENT COMPLETION OF THE OTHER CONTROL CIRCUIT, EACH OF SAID CONTROL RELAYS HAVING NORMALLY OPEN SWITCH CONTACTS ARRANGED, WHEN CLOSED, TO BYPASS SAID TIME DELAY RELAY SWITCH CONTACTS OF ITS OWN CONTROL CIRCUIT, WHEREBY ACTUATION OF ONE OF SAID TIME DELAY RELAYS MAY NOT OPEN THE OTHER CONTROL CIRCUIT AFTER ACTUATION OF THE CONTROL RELAY THEREOF, SAID OUTPUT CIRCUIT INCLUDING, INSERIES ARRANGEMENT, A LOAD DEVICE AND TWO NORMALLY OPEN SWITCHES, EACH OF SAID SWITCHES BEING ASSOCIATED WITH ONE OF SAID CONTROL CIRCUITS AND BEING CLOSABLE BY ACTUATION OF ONE OF THE RELAYS THEREOF. 